Chicago ? An experimental treatment added four months to the lives of men with advanced prostate cancer in a study that tested an entirely new approach to fighting the disease, doctors reported Tuesday.

Dendreon Corp.’s Provenge vaccine trains the immune system to fight tumors. It’s called a “vaccine” even though it treats disease rather than prevents it.

Doctors have been trying to develop such a therapy for decades, and this is the first to meet a preset goal for improving survival in late-stage testing.

“There have been a lot of false starts, but this is a real start,” said Dr. Paul Schellhammer, a urologist at Eastern Virginia Medical School in Norfolk, Va., who led the study. Results were reported Tuesday at an American Urological Association conference in Chicago.